This application is based upon and claims the benefit of priority of Japanese Patent Applications No. H.9-153746 filed on Jun. 11, 1997, No. H.9-321022 filed on Nov. 21, 1997 and No. H.10-119089 filed on Apr. 28, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a method for manufacturing a semiconductor substrate having a pressure reference chamber therein, used for a pressure sensor or the like and relates to a semiconductor pressure sensor and a manufacturing method thereof.
2. Related Art
Semiconductor pressure sensors for detecting the pressure acting on a diaphragm include those constructed with a pressure reference chamber provided thereinside. In this case, in order to maximize the detecting accuracy of the sensor, the amount of gas remaining inside the pressure reference chamber is made as small as possible to reduce fluctuations of the reference pressure inside the pressure reference chamber resulting from temperature variations.
As semiconductor substrates used in the manufacture of this kind of semiconductor pressure sensor, substrates wherein a part corresponding to a pressure reference chamber is formed in advance have been used. This kind of semiconductor substrate is made for example by laminating together two silicon substrates so as to form a pressure reference chamber thereinside, as shown in FIGS. 23A through 23C.
That is, first, as shown in FIG. 23A, a concavity 2 used as a pressure reference chamber is formed in a first silicon substrate 1 by a method such as etching. Also, an oxide film 4 is formed on the surface of a second silicon substrate 3. Then, the first silicon substrate 1 and the second silicon substrate 3 are laminated together so that the concavity 2 in the first silicon substrate 1 is covered by the face of the second silicon substrate 3 on which the oxide film 4 is formed. This lamination is carried out in a vacuum. As a result, in the laminated state, the concavity 2 is covered by the second silicon substrate 3 and forms a pressure reference chamber 5 containing a vacuum (see FIG. 23B).
Then, by polishing the exposed face of the first silicon substrate 1, the thickness of the bottom part of the pressure reference chamber 5 is brought to a predetermined thickness to form a part to become a diaphragm 6. After that, a plurality of resistors having a piezoresistance effect are formed in the diaphragm 6 and these are connected in the form of a bridge circuit to complete a semiconductor pressure sensor.
When the pressure of an environment in which the semiconductor pressure sensor has been placed acts on the diaphragm 6, the diaphragm 6 is displaced by a force corresponding to the difference between this pressure and the pressure inside the pressure reference chamber 5. In correspondence with this displacement of the diaphragm 6, the resistances of the resistors are changed by a piezoresistance effect. At this time, a voltage corresponding to the pressure of the environment is outputted to output terminals of the bridge circuit, and by detecting this output voltage it is possible to detect the pressure.
However, this kind of semiconductor pressure sensor detects the pressure acting on the diaphragm 6 as a change in the resistances of resistors changing in correspondence with the displacement of the diaphragm 6. Consequently, the thickness dimension of the diaphragm 6 is a factor determining the precision of the pressure detection. That is, if the diaphragm 6 is made thin, the detection precision can be increased correspondingly. And, to reduce the area of the diaphragm 6 without decreasing the detection precision it is necessary to make the thickness of the diaphragm 6 thin.
However, with the kind of semiconductor substrate manufacturing method described above, after the pressure reference chamber 5 is formed with its interior nearly at a vacuum state it is necessary for a polishing step to be carried out to form the diaphragm 6. But when polishing progresses so as to make the diaphragm 6 thin (for example about 1 to 10 xcexcm), during polishing the diaphragm 6 undergoes stress due to the pressure difference between the inside of the pressure reference chamber 5 and the outside and deforms as shown in FIG. 23C.
When deformations of the diaphragm 6 during polishing becomes large as much as not to be negligible, the thickness of the diaphragm 6 formed is uneven and the accuracy of the displacement of the diaphragm 6 corresponding to the pressure being detected falls. And, in some cases, the central part of the diaphragm 6 may come into contact with the opposite wall of the pressure reference chamber 5 so that further displacement of the diaphragm 6 is obstructed.
It is therefore an object of the present invention to provide a method for manufacturing a semiconductor substrate which, when a diaphragm constituting the rear wall of a pressure reference chamber is processed to a low thickness, does not adversely affect the diaphragm due to stress causing the diaphragm to deform, which is generated based on the pressure difference between the inside and the outside of a pressure reference chamber and to provide a semiconductor pressure sensor and the manufacturing method thereof.
According to a first aspect of the invention, in a concavity forming step a concavity is formed in a first substrate, and in a laminating step the first substrate is laminated with a second substrate in an atmosphere at atmospheric pressure and the concavity is thereby formed as a pressure reference chamber, after which the pressure reference chamber is evacuated in an evacuating step. The laminating step does not have to be carried out in an evacuated atmosphere, and therefore the laminating step can be carried out simply and easily. Furthermore, problems such as the substrate being deformed by a pressure difference between the inside and the outside of the pressure reference chamber do not arise when the thickness of a part of the substrate where the pressure reference chamber is formed is processed by polishing or the like or a step of forming devices there is carried out prior to executing the evacuating step, and consequently this processing can be carried out with good precision.
Preferably, in a connecting hole forming step, a connecting hole is formed in at least one of the first and second substrates so that when the first and second substrates have been laminated together the concavity to be used as a pressure reference chamber is connected to the outside. When this is done, after the laminating step is carried out, in carrying out the evacuating step, if the gas inside the pressure reference chamber is removed through this connecting hole and a sealing step is carried out to close the connecting hole after the evacuation, the pressure reference chamber can be evacuated surely.
The evacuating step is preferably carried out after a device forming step of forming devices constructing a pressure sensor is carried out. By this means, when a part of the substrate in which the pressure reference chamber is formed is used as a diaphragm, the diaphragm can be prevented from deforming due to a pressure difference across it while the substrate is being processed or the devices are being formed. Therefore, restrictions on the processing for diaphragm formation or device formation can be reduced and the accuracy of this processing can be increased.
In the laminating step, the concavity used as a pressure reference chamber may be sealed so that the inside thereof is isolated from the outside, and then in the evacuating step, by heat treatment being carried out, the gas inside the concavity may be made to combine with the substrate material and thereby consumed. By this means, it is possible to evacuate the inside of the pressure reference chamber simply and surely. In this evacuating step, oxygen remaining inside the concavity is combined with the substrate material to form an oxide, and as a result the inside of the pressure reference chamber is evacuated. Consequently, evacuation of the inside of the pressure reference chamber can be carried out simply and surely without using special reactants or the like.
To promote the above-mentioned reaction, the inside of the concavity may be surface-treated in advance so that semiconductor faces are exposed. In this case, the evacuating step can be carried out efficiently. As this surface treatment, an oxide film may be removed. When this is done, because semiconductor faces consume oxygen inside the pressure reference chamber more readily, the evacuating step can be carried out surely and rapidly.
When polishing is carried out to form a diaphragm by reducing the thickness of the part of the first substrate where the concavity used as a pressure reference chamber is formed, the polishing step is preferably carried out before the evacuating step. When this is done, the part being formed into a diaphragm does not deform under stress arising due to a pressure difference across it, and consequently a diaphragm used in a pressure sensor or the like can be formed with good accuracy.
Preferably, the thickness of the diaphragm is determined so that a maximum value of deflection which is derived from the thickness, a side length, an elastic modulus determined from a material of the diaphragm and a pressure uniformly acting on the surface of the diaphragm is equal to or lower than the thickness of the diaphragm. In this case, even when the thickness of the diaphragm is set thin, it is possible to reduce the deformation of the diaphragm cased by a pressure difference between the inside and the outside of a pressure reference chamber. Therefore, during a manufacturing process, even when the part of the substrate to become the diaphragm is polished to form the diaphragm after the inside of the pressure reference chamber is evacuated, problems caused by the deflection and deformation of the diaphragm can be suppressed and therefore, the diaphragm causing no obstacle for pressure detection can be obtained.
Preferably, the shape of the diaphragm is square. Further, when the diaphragm is formed from monocrystalline silicon, the ratio (L/H) of a side""s length L to a thickness H of the diaphragm is preferably set to be smaller than 104. By this means also, even when the part of the substrate to become the diaphragm is polished after the inside of the pressure reference chamber is evacuated, problems caused by the deflection and deformation of the diaphragm can be suppressed and therefore, the diaphragm causing no obstacle for pressure detection can be obtained.
The shape of the diaphragm may be circle. In this case, the thickness of the diaphragm can be determined so that a maximum value of deflection which is derived from the thickness, a radius and an elastic modulus determined from a material of the diaphragm and a pressure uniformly acting on the surface of the diaphragm is equal to or lower than the thickness of the diaphragm. Also, when the diaphragm is formed from monocrystalline silicon, the ratio (R/H) of a radius R to a thickness H of the diaphragm is preferably set to be smaller than 56.
In these cases, even when the thickness of the diaphragm is set thin, it is possible to reduce the deformation of the diaphragm cased by a pressure difference between the inside and the outside of a pressure reference chamber. Therefore, during a manufacturing process, even when the part of the substrate to become the diaphragm is polished to form the diaphragm after the inside of the pressure reference chamber is evacuated, problems caused by the deflection and deformation of the diaphragm can be suppressed and therefore, the diaphragm causing no obstacle for pressure detection can be obtained.
A second aspect of the invention provides a method for manufacturing a semiconductor pressure sensor substrate. That is, the invention provides a method for manufacturing a semiconductor pressure sensor substrate having a pressure reference chamber on the rear side of a diaphragm, the method comprising: a concavity forming step of, by etching a surface of an active layer provided on a first supporting substrate with a PN junction therebetween, forming a concavity to become a pressure reference chamber leaving an active layer of a thickness corresponding to that of the diaphragm provided on its bottom side; a laminating step of laminating a second supporting substrate to the surface of the active layer; a first supporting substrate removing step of removing the most part of the first supporting substrate by etching; and a finishing etching step of further carrying out shallow etching on the etching face of the first supporting substrate removing step until the active layer becomes exposed. The first supporting substrate removing step is carried out using electrochemical stop etching with a depletion layer formed at the PN junction between the first supporting substrate and the active layer as a stopper.
Because the electrochemical stop etching used in the first supporting substrate removing step is carried out with a depletion layer formed at the PN junction between the first supporting substrate and the active layer as a stopper, there is no occurrence of the kind of sagging that arises when the first supporting substrate is removed by polishing, and the etching can be carried out leaving the first supporting substrate of a uniform thickness on the active layer. In the finishing etching, it is only necessary to carry out extremely shallow etching for removing the remaining depletion layer part of the first supporting substrate. Consequently, thickness variation caused by the finishing etching is small enough to be negligible. As a result, it is possible to form a diaphragm having extremely little thickness variation.
Therefore, with this second aspect of the invention, it is possible to obtain a diaphragm of a uniform film thickness, troublesome steps such as trench processing are unnecessary, and there is the highly valuable benefit that the process is simple and manufacturing costs can be made low. And, because the diaphragm and the concavity to become the pressure reference chamber are formed in the active layer, the thickness of the active layer can be made large irrespective of the thickness of the diaphragm, and thus the freedom of circuit design is increased. Also, positional alignment in the laminating step can be made unnecessary.
The active layer can be formed on the first supporting substrate by epitaxial growth. At this time, a concave or projecting alignment mark can be preformed in the upper face of the first supporting substrate. When this is done, a concave or projecting alignment mark corresponding to the alignment mark preformed in the first supporting substrate also appears in the surface of the active layer formed on the first supporting substrate. Then, in the concavity forming step, the concavity can be formed in a predetermined position on the basis of that alignment mark. Also, because after the finishing etching step an alignment mark appears at the junction between the first supporting substrate and the active layer, that is, at the exposed surface of the active layer, subsequent formation of devices and circuits can be carried out in predetermined positions in the active layer on the basis of this alignment mark.
In the first supporting substrate removing step, electrochemical stop etching to the depletion layer may be carried out after the first supporting substrate is partially removed by polishing. When this is done, as a result of polishing also being used, the amount of material to be removed by the electrochemical stop etching is reduced. Consequently, the process time can be shortened and cost reductions can be achieved with it still being possible to carry out uniform etching.
Also, by the laminating step being carried out in a vacuum, the pressure reference chamber can be made a vacuum chamber.